1. Field of the Invention
This invention relates to an electro-luminescence display (ELD), and more particularly to an electro-luminescence display device and a driving apparatus thereof that prevents the deterioration of a thin film transistor to enhance image quality of the ELD.
2. Description of the Related Art
Recently, various flat panel display devices have been developed that eliminate disadvantages of a cathode ray tube (CRT) by reducing the weight and bulk of the display. Such flat panel display devices include a liquid crystal display (LCD), a field emission display (FED), a plasma display panel (PDP) and an electro-luminescence (EL) display, etc.
The EL display is a self-luminous device capable of emitting light by a re-combination of electrons with holes using a phosphorous material.
The EL display device is generally classified as an inorganic EL device using the phosphorous material in an inorganic compound or an organic EL device using the phosphorous material in an organic compound. EL display devices have many advantages such as a low voltage driving, self-luminescence, a thin profile, a wide viewing angle, a fast response speed, a high contrast, etc.
The organic EL device includes an electron injection layer, an electron carrier layer, a light-emitting layer, a hole carrier layer and a hole injection layer. In the organic EL device, when a predetermined voltage is applied between an anode and a cathode, electrons produced from the cathode are moved, via the electron injection layer and the electron carrier layer, into the light-emitting layer while holes produced from the anode are moved, via the hole injection layer and the hole carrier layer, into the light-emitting layer. Thus, the electrons and the holes fed from the electron carrier layer and the hole carrier layer into the light-emitting layer emit light by the recombination of the electrons and the holes.
Referring to FIG. 1, an active matrix type EL display device using the above-mentioned organic EL device includes an EL panel 20 having pixels 28 arranged at each crossing between gate lines GL and data lines DL, a gate driver 22 driving the gate lines GL of the EL panel 20, a data driver 24 driving the data lines DL of the EL panel 20, and a gamma voltage generator 26 supplying the data driver 24 with a plurality of gamma voltages.
The gate driver 22 applies a scanning pulse to the gate lines GL to sequentially drive the gate lines SL. The data driver 24 converts a digital data signal received from an external signal source into an analog data signal using a gamma voltage from the gamma voltage generator 26. The data driver 24 applies the analog data signal to the data lines DL whenever the scanning pulse is supplied.
Each of the pixels 28 receives a data signal from the data line DL and generates light corresponding to the data signal when a scan pulse is applied to a gate line GL.
As shown in FIG. 2, each pixel 28 includes an EL cell OEL having an anode connected to the voltage source VDD and a cathode connected to a cell driver 30, a gate line GL, a data line DL and a ground voltage source GND to drive the EL cell OEL.
The cell driver 30 includes a switching thin film transistor T1 having a gate terminal connected to the gate line GL, a source terminal connected to the data line DL and a drain terminal connected to a first node N1, a driving thin film transistor T2 having a gate terminal connected to the first node N1, a drain terminal connected to the ground voltage source GND and a source terminal connected to the EL cell OEL, and a storage capacitor Cst connected between the ground voltage source GND and the first node N1.
The switching thin film transistor T1 is turned on when the scan pulse is applied to the gate scan line GL, to thereby apply the data signal, which is supplied from the data line DL, to the first node N1. The data signal supplied to the first node N1 is charged into the storage capacitor Cst and is applied to the gate terminal of the driving thin film transistor T2. The driving thin film transistor T2 controls a current I fed from the voltage source VDD via the EL cell OEL in response to the data signal applied to the gate terminal thereof, to thereby control the amount of light emitted from the EL cell OEL. Furthermore, even though the switching thin film transistor T1 is turned off, the driving thin film transistor T2 maintains an on state because of the data signal charged in the storage capacitor Cst and thus continues to control the current I from the voltage source VDD via the EL cell OEL until a data signal is supplied at the next frame.
The current I flowing to the EL cell OEL may be represented as a formula 1.
                    I        =                              W                          2              ⁢              L                                ⁢                                    Cox              ⁡                              (                                                      Vg                    ⁢                                                                                  ⁢                    2                                    -                  Vth                                )                                      2                                              [                  Formula          ⁢                                          ⁢          1                ]            
Herein, W represents the width of a driving thin film transistor T2 and L represents the length of the driving thin film transistor T2. Cox represents the capacitor value provided by a dielectric layer which forms one layer. Further, Vg2 represents the voltage value of the data signal applied to the gate terminal of the driving thin film transistor T2 and Vth represents a threshold voltage value of the driving thin film transistor T2.
In the formula 1, parameters W, L, Cox and Vg2 do not vary as the thin film transistor T2 ages.
However, the driving thin film transistor deteriorates due to a continuous supply of a positive voltage to the gate terminal, and its current driving scheme. A threshold voltage of the driving thin film transistor increases over time because the driving thin film transistor deteriorates. As described above, if the threshold voltage of the driving thin film transistor is increased, the amount of current flowing to the EL cell OEL can not be accurately controlled, so that the luminance is decreased and the desired image is not displayed.
The driving thin film transistor T2 is made using hydrogenated amorphous silicon. Hydrogenated amorphous silicon has an advantage in that it is easy to manufacture in a large scale and it is possible to the deposit at low temperatures less than 350° C. Thus, the thin film transistors are typically made using the hydrogenated amorphous silicon.
However, in hydrogenated amorphous silicon there exists a Weak Si—Si bond 32 and a dangling bond as shown in FIG. 3 because of a disordered atom array. Over time, an Si bonded by the Weak Si—Si bond migrates from the atom array as shown in FIG. 3B, and thus the electrons and the holes recombine in the site from which the Si has migrated, or a migration state remains. The change in the atomic structure of the hydrogenated amorphous silicon induces a change of energy level such that the threshold voltage Vth of the driving thin film transistor is increased to the values of Vth′, Vth″, Vth′″ as shown in FIG. 4. Consequently, due to the increased threshold voltage of the driving thin film transistor, it is difficult to accurately represent the brightness of the image as desired in the EL panel 20. Furthermore, a small decrease of the brightness results in an residual image in the EL panel 20, thereby adversely effecting image quality.